The main supply pumps in transmissions are generally geared directly to the engine, and the output flow of such supply pumps is, therefore, proportional to the speed of the engine. The flow requirements of the transmission, on the other hand, are not generally a function of engine speed. Accordingly, the supply pump for an automatic transmission has heretofore been sized to provide the transmission flow requirement at engine idle speed. This design approach results in excess capacity at elevated engine speeds. Thus, a considerable amount of power is wasted at higher pump speeds when the excess fluid must be exhausted through a system relief valve.
The prior art systems have attempted to reduce this loss by using variable displacement vane pumps and two stage pumps. While variable displacement pumps will reduce the power requirements at high speeds, such pump assemblies are considerably more expensive than two stage pumps. For that reason the most common solution has been to adopt and use two stage pumps. In two stage pump mechanisms, the first stage pump provides continuous fluid flow whenever the engine is operating. On the other hand, the second stage pump flow is directed into the main circuit, or dumped to sump, depending on the requirements of the transmission. Dumping a large amount of oil to sump still requires a significant amount of power. To reduce the power requirement of the second stage even further, the pump inlet is connected to atmosphere in order to ingest air into the pump instead of oil. This solution is noisy, complicates the control mechanism and retards the response of the second stage pump when its fluid flow is required from the second stage pump.